1. Field of the Invention
The present invention relates to oxide ion conductors which are effectively used for electrolytes or air electrodes for fuel cells, gas sensors such as oxygen gas sensors, oxygen separation membranes for electrochemical oxygen pumps and the like, gas separation membranes, and the like.
2. Description of the Related Art
As a typical example of conventional oxide ion conductors, a solid solution having a cubic fluorite system is known as “a stabilized zirconia” in which a small amount of a divalent or a trivalent metal oxide, such as CaO, MgO, Y2O3, or Gd2O3, is dissolved in zirconium oxide (ZrO2). A stabilized zirconia has superior heat stability, and in addition, has an advantage in which the ionic transference number (a ratio of oxide ionic conduction to electrical conduction) does not tend to decrease even if the oxygen partial pressure is decreased since the oxide ion conduction is dominant at all oxygen partial pressures from an oxygen atmosphere to a hydrogen atmosphere. Accordingly, a stabilized zirconia is widely used as zirconia (oxygen) sensors for various industrial process controls, such as for steel manufacturing, and for combustion control (an air-fuel ratio) for automobiles. In addition, a stabilized zirconia is also used as an electrolyte for a solid oxide fuel cell (SOFC) under development, which is operated at approximately 1,000° C.
However, the oxide ionic conduction of a stabilized zirconia is not sufficiently high, and in particular, the conduction thereof becomes deficient when a temperature is decreased. For example, the ionic conductivity of Y2O3— stabilized zirconia is 10−4 S/cm at 1,000° C. but is decreased to 10−4 S/cm at 500° C., whereby there is an inconvenient limitation in which the operating temperature must be controlled at a higher temperature, such as 800° C. or more.
In order to solve the problems described above, an oxide ion conductor having a perovskite structure is proposed provided with oxide ionic conduction higher than that of a stabilized zirconia (refer to Japanese Unexamined Patent Application Publication Nos. 11-228136, 11-335164). These oxide ion conductors mentioned above are compound oxides composed of four elements or five elements, and an oxide ion conductor disclosed in Japanese Unexamined Patent Application Publication No. 11-335164 is a substance represented by the formula Ln1-xAxGa1-y-zB1yB2zO3 in which Ln is a lanthanoid rare earth metal, A is an alkaline earth metal, B1 is a non-transition metal, and B2 is a transition metal. That is, this oxide ion conductor has a basic lanthanoid.gallate (LnGaO3) structure and is a compound oxide composed of five elements (Ln+A+Ga+B1+B2) formed by doping three elements, i.e., an alkaline earth metal (A), a non-transition metal (B1), and a transition metal (B2), in the lanthanoid.gallate structure, or is a compound oxide composed of four elements (Ln+A+Ga+B2) formed by doping two elements, i.e., an alkaline earth metal (A), and a transition metal (B2), in the lanthanoid.gallate structure.
The oxide ion conductor described above has oxide ionic conduction higher than that of a stabilized zirconia and has superior heat stability, in which the high oxide ionic conduction thereof can be maintained at a higher temperature and also even at a lower temperature. Furthermore, it is confirmed that the decrease in ionic transference number is preferably small at all oxygen partial pressures from an oxygen atmosphere to a hydrogen atmosphere (i.e., even at a lower oxygen partial pressure), and that oxide ionic conduction is dominant, or mixed ionic conduction is observed.
However, in the oxide ion conductor disclosed in Japanese Unexamined Patent Application Publication No. 11-228136, there is a problem in that the oxide ionic conduction is low, and in the oxide ion conductor disclosed in Japanese Unexamined Patent Application Publication No. 11-335164, there is a problem, which must be overcome, in that the mechanical strength is not sufficient. Since an oxide ion conductor is used in a manner in which gases having different compositions from each other are supplied at the front and the rear surfaces of the oxide ion conductor, respectively, to contact thereon so that reactions occur, when cracks or continuous pores are formed in the oxide ion conductor, the gases at the front and the rear surfaces thereof leak through the cracks or the continuous pores. When the gases leak, the performance of the component is decreased, the efficiency thereof is significantly degraded, and in addition, the entire component may be seriously damaged.